He got 46% of the vote nationally and 56% in Missouri in 2016. He's now at 44% approval nationally. Are you just ignoring all the recent polls and pretending he's still at 40% approval or something?

2016 results are a universe of their own. Your statement would be more relevant if Trump won completely on his own merits in 2016, as opposed to a significant chunk of the vote not only voting against Clinton, but many even voting 3rd party.

In this sense, it's perfectly plausible for Trump to be slightly more popular in 2020 yet win Missouri by significantly less, or even lose it entirely, so long as his Democratic opponent is semi-popular and not a total dud. It's also shouldn't be surprising for MO Republicans to do worse this cycle despite Trump being more popular than he was in 2016, for the aforementioned reasons.

My point isn't about 2020 or Trump's opponent though. It's just the simple fact that if Trump has a 44% approval rating nationally, then it has to be coming from somewhere. And considering Missouri is a red state that he won by 18 points it would only make sense that it is one of the states where he is doing significantly better than his overall approval, unless he's surging in places like California and Oklahoma to make up the difference. Or the polls are just wrong. But Trump being above 50% in Missouri is certainly logically consistent with him being at 44% nationally.